The present invention relates to acoustic sensors in which a diaphragm, in response to incident acoustic waves, modulates light intensity in an optical interferometer.
Acoustic sensors employing optical techniques can be grouped in two major classes: intensity and interferometric. Intensity-based acoustic sensors vary the intensity of a continuous wave (CW) laser, or other light source, typically by disposing in the laser beam a variable optical attenuator that responds to acoustically induced motions of a diaphragm. Such devices are described in U.S. Pat. Nos. 4,458,343 to Tehon et al. and No. 4,310,905 to Palmer.
Conventional intensity-based sensors are approximately three orders of magnitude less sensitive than interferometric-based sensors, although the variable-ratio fiber-optic coupler hydrophone approaches the latter type's sensitivity. The variable-ratio coupler hydrophone utilizes a single-mode, fiber optic coupler as the sensing element and is thus generally rugged and sensitive. One such sensor is described in U.S. Pat. No. 4,545,253 to Avicola. Nevertheless, the variable-ratio coupler hydrophone suffers a major drawback for use under high pressure: it is extremely difficult to pressure compensate. Accordingly, such a coupler would not be a good candidate for use in environments such as the deep sea.
The interferometric-based acoustic sensor class can be divided into two common sensor types: fiber coil sensors and diaphragm sensors; both types can be engineered to achieve excellent sensitivity to acoustic pressure. Fiber coil sensors, which generally rely on interferogram modulation by pressure-induced refractive index changes in fiber coils, are described in U.S. Pat. Nos. 4,320,475 to Leclerc et al.; No. 4,593,385 to Chamuel; No. 4,743,113 to Jubinski and No. 4,799,752 to Carome. Nevertheless, fiber coil sensors tend to be relatively heavier and have larger parts count and cost than diaphragm sensors, and pressure compensation poses serious problems.
The fiber coil sensor used in the all-optical towed array described in U.S. Pat. No. 4,648,083 to Giallorenzi employs an air-backed compliant mandrel in its pressure compensation scheme. Present implementations of the air-backed mandrel design are limited in the pressure range over which linear operation can be obtained and are subject to permanent damage from pressure overloads. In addition, the air-backed mandrel, fiber coil hydrophone using a Mach-Zehnder or Michelson interferometer configuration is costly because of its many precision-machined components, large amount of fiber and many optical beam splitters.
An elementary diaphragm-type interferometric-based acoustic sensor is disclosed in U.S. Pat. No. 4,446,543 to McLandrich et al. Light from a laser is transmitted by an optical fiber to an optical resonator comprising the partially reflective end of the fiber and a mirror-diaphragm. The mirror moves with respect to the fiber end in response to an acoustic signal, thereby modulating the intensity of the combined reflections from the fiber end and the mirror. The reflected light is directed to a photodetector that produces an electrical signal representative of the interference pattern. Apart from the basic features of a diaphragm-type acoustic sensor, the McLandrich et al. patent fails to show methods for pressure compensating the sensor, for demodulating the reflected light that contains the acoustic information and for multiplexing the outputs of several sensors on an optical bus.
The above-cited patent to Tehon et al. shows one method of pressure-compensating a diaphragm-type hydrophone. A chamber behind the sensing diaphragm is filled with oil and communicates with a second oil-filled chamber through a capillary tube. The second chamber is subject to the ambient pressure through a second diaphragm. The capillary tube, which is sometimes called a Helmholtz tube, equalizes the hydrostatic pressure between the chambers but permits dynamic pressure differences over the frequency range of interest. To compensate for the low compliance of the oil, which would severely reduce the hydrophone's sensitivity, the Tehon et al. hydrophone includes a compliant bellows. Pressure-compensation techniques, as well as general considerations related to hydrophones, are also described in the "Handbook of Hydrophone Element Design Technology," C. LeBlanc, NUSC Technical Document No. 5813, Naval Underwater Systems Center (Oct. 11, 1978).
Among prior methods for multiplexing the outputs of a plurality of acoustic sensors on an optical fiber are the FM phase generated carrier and time-division-multiplexed phase generated carrier methods, the path matched differential interferometry technique, and the wavelength tuning method. The phase generated carrier technique is described in "Homodyne Demodulation Scheme for Fiber Optic Sensors Using Phase Generated Carrier," A. Dandridge et al., IEEE Jour. Quantum Elecs., vol. QE-18, pp. 1647-1653 (October, 1982) . The path matched differential interferometry technique is described in "Fiber Optic Interferometric Sensor Arrays with Freedom from Source Induced Noise," J. Brooks et al., Optics Letters, vol. 11, pp. 473-475 (July, 1986). The wavelength tuning technique is described in "Stability Improvement of Fiber Optic Probe Microphone by Frequency Control of the Light Source," R. Ohba et al., Jour. Physics E. Sci. Instrum. 20, pp. 1380-1382 (1987). A survey of these and other methods is provided in "Overview of Multiplexing Techniques for Interferometric Fiber Sensors," A. Kersey et al., Proc. SPIE, vol. 838 Fiber Optic and Laser Sensors V, pp. 184-193 (1987).